1. Technical Field
The invention relates to image scanning systems. More particularly, the invention relates to a long depth of focus contact image sensor module for use in compact and light weight flatbed type scanning systems.
2. Description of the Prior Art
Historically, linear image sensor arrays, in a form of charge-couples device (CCD's) or self-scanned photodiode arrays (or MOS arrays), have been used to accomplish image scanning. Early prior art scanning devices required at least three essential elements to capture the image of a subject document: (1) a light source to illuminate the document, (2) an image sensor array to detect the image signal from the reflected document, and (3) a lens system to focus the image of the document on the image sensor.
Two image sensor systems had been widely used to scan a page width of a document image. They are the lens reduction image sensor system and the contact type image sensor system.
The Charge Coupled Device (CCD) image sensor has been used in the lens reduction image sensor system for more than twenty years. Because the length of a CCD image sensor is about one inch, a concave reduction lens is required to focus the image of a standard A4 paper size on the CCD array. A plastic case is used to house all of the components (e.g. CCD image sensor, reduction lens, and necessary optical path) of a CCD module. This CCD module is quite bulky and heavy because the optical distance is long (about 10-30 centimeter). Moreover, the whole module must be moved by a high power stepping motor, which typically produces jerky or uneven movement of the module, causing distortion in the image. Overall, the flatbed scanner is large and heavy.
About ten years ago, the contact type image sensor (CIS) system was designed to solve the aforementioned problem. In this system, the reduction lens is replaced by a Selfoc Lens Array (SLA) and the CCD image sensor is replaced by a full width, hybrid image sensor array. The optical distance is reduced to less than one centimeter. This system is then assembled into a compact and light weight module, i.e. a CIS module. The module has all of the elements required to capture an image.
One drawback of the CIS module is its short Depth Of Focus (DOF). The CIS module uses an SLA array to focus the document image to the image sensor. Because the DOF of the SLA array is about 0.3 millimeters, the DOF of the CIS module is about 0.3 millimeters. This problem limits the CIS's application to paper feeder type image scanning systems. The CIS module can not used on flatbed type image scanning systems because such systems need a very long DOF on the document plane.
A typical configuration of a prior art scanning device (i.e. a lens reduction image sensor system) is illustrated in FIG. 1. An original document 1 is illuminated by a light source 2. Because an ordinary CCD image sensor 3 is typically approximately one inch long, an optical lens 4 is required to reduce the image of any text or graphics on the document 1 so that a full width (about 8.5 inch) image can be received in the CCD image sensor 3. The reduction rate of an optical lens 4 depends on what kind of CCD image sensor 3 is used in the system. The optical distance ("A" cm) from the document 1 to the lens 4 is about seven to eight times the optical distance ("B" cm) from the image sensor 3 to the lens 4. Therefore, the depth of focus (DOF) on the document 1 is about seven to eight times longer than the DOF on the image sensor 3.
Because this system has a very long DOF, most of the early image scanner (both paper feeder type and flatbed type) used a lens reduction image sensor system. To obtain the necessary reduction, an optical distance of about 10 to 30 centimeters is required between the CCD image sensor 3 and the document 1. This optical separation distance necessitates a rather bulky assembly for the overall scanning device. For this reason, some prior art devices use sophisticated (hence expensive and difficult to make) folded optical schemes that add several mirrors 5 to reduce the total physical size of the assembly.
FIG. 2 shows a perspective view of a folded optical scheme for a lens reduction image sensor system. A plastic case is used to house all of the components together (e.g. CCD image sensor 3, reduction lens 4, mirrors 5, and necessary optical path) to a CCD module. The whole CCD module of the folded optical schemes or unfolded optical scheme is big, heavy, and bulky.
An improvement to the image sensor system (as discussed above, see FIG. 3) is the CIS system. In this system, the reducing optical lens is replaced by a full width Selfoc Lens Array (SLA) 6 and the CCD image sensor is replaced by a full width image sensor array 7. This system allows a one-to-one scanning of the document 1 because the SLA 6 and a full width image sensor array 7 are of the same width as the document 1 to be scanned. This arrangement allows the distance between the image sensor array 7 and the document 1 to be reduced to less than one centimeter.
A CIS module is used to integrate all of the components to a compact and light weight module. FIG. 4 is a cross section view of the prior art CIS module. The CIS module consists of: (1) a LED light source 2 to illuminate the document 1, (2) a one-to-one Selfoc Lens Array (SLA) 6 to focus the document image 1 to the image sensor array 7; (3) a hybrid image sensor board 8 which comprises a plurality of image sensor arrays 7 and its associated circuitry is used to convert an optical signal to an electronic signal; (4) a cover glass 9; (5) a connector 10 to connect the electronic signal of the module to outside circuitry; and (6) a case 11 to house all of the above components. The overall dimension of the CIS module 12 is typically less than 18.times.13.times.232 mm for A4 size paper width. The CIS module's size is small relative to that of the CCD module.
Therefore, the overall dimensions of an image scanner which incorporates a CIS module 12 are smaller than those of an image scanner that incorporates a CCD module.
Current CIS modules have a very short DOF (about 0.3 millimeter) due to the short DOF on the SLA. Normally, the DOF of the current SLA used on the CIS module is about 0.3 millimeter. This short DOF problem limits the CIS module's application on paper feeder type scanners (for example, fax machines) as opposed to flatbed type scanners (for example, copiers). FIG. 5 shows a paper feeder type image scanning system in which the CIS module 12 is fixed and the document 1 is moved by the roller 13. The document 1 is sandwiched between the roller 13 and the glass 9 surface of the CIS module 12. When the stepping motors drive the roller 13, the document 1 is moved forward simultaneously. The document 1 is pushed by the roller 13 and contacts the glass 9 surface directly. The focus point of the CIS module is adjusted to the surface of the glass 9. In this case, the image sensor array 7 is able to detect the document image 1 and prevent the short DOF problem on the CIS module.
Most paper feeder image scanning systems (for example, fax machines and sheet fed scanners) currently use CIS modules instead of CCD modules because of the CIS module's compact size and ease of incorporation into a system assembly. Paper feeder scanning systems only scan a sheet of document. A bound document (for example, a book or magazine) can not be scanned using a paper feeder scanner.
One image scanner that is able to scan a book or magazine is a flatbed type image scanning system (see FIGS. 6a and 6b). On the flatbed type image scanning system (for example, a flatbed scanner or digital copier), the document 1 is fixed between a cover plate 14 and a glass 15 surface of the flatbed scanner. The CCD module 16 is moved underneath the glass 15 forward to detect the document image 1. Normally, glass thickness variation across the whole page width of the glass plate 15 is more than 0.3 millimeter. The CCD module 16 is guided by two long stainless steel, spherical railways 17 at both ends of the CCD module 16. The two long, spherical railways 17 coupled to the CCD module 16 act like a railroad to a train. The CCD module 16 is moved forward by a gear belt which controlled by a stepping motor. The whole module has about a few tenths of a millimeter of vibration up and down on its optical path direction (vertical direction of FIGS. 6a and 6b) when the CCD module 16 is moved forward.
In addition, the document 1 that sits on the glass 15 surface is not optically flat and has a few tenths of a millimeter of surface variation. In many cases, people want to scan a book or magazine. Normally, the book or magazine has a curved surface or rough surface. When the total variation is added together, there must be at least a few millimeter of DOF for the optical distance on the flatbed scanner to detect a decent image signal on the image sensor array. A longer DOF is always better for flatbed scanner. Therefore, current CIS modules can not be used on flatbed type image scanning systems.
Flatbed scanners require a very long DOF to detect a decent image signal. Current flatbed scanners use a CCD module to capture an image because such module provides a long DOF on the document plane. Because of such module's long (10 to 30 centimeter) optical distance, current flatbed scanners are bulky and heavy (more than 10 inch wide, 15 inch long, and 4 inch high). In the current office environment, most of the workers have a computer, monitor, CD-ROM, and printer on their desk. There is not enough space to put a scanner on most of the worker's desks. For a flatbed scanner to gain the acceptance of a CD-ROM or printer in the office environment, the sizes of the flatbed scanner need to be compact and light weight, and the scanner must have a low cost. The prior art CIS module might be a candidate to reduce the size of the flatbed scanner but, because of its short DOF problem, current CIS modules cannot used on the flatbed scanner.